Multi-nuclear, high-pressure, operando FlowNMR spectroscopic study of Rh/PPh3 – catalysed hydroformylation of 1-hexene

• Published in: Faraday discussions 2021-06, Vol.229, p.422-442
• Main Authors: Bara-Estaún, Alejandro, Lyall, Catherine L, Lowe, John P, Pringle, Paul G, Kamer, Paul C J, Franke, Robert, Hintermair, Ulrich
• Format: Article
• Language: English
• Subjects: Carbon monoxide; Catalysis; Isomers; Phosphines
• ISSN: 1359-6640; 1364-5498
• DOI: 10.1039/c9fd00145j

The hydroformylation of 1-hexene with 12 bar of 1 : 1 H2/CO in the presence of the catalytic system [Rh(acac)(CO)2]/PPh3 was successfully studied by real-time multinuclear high-resolution FlowNMR spectroscopy at 50 °C. Quantitative reaction progress curves that yield rates as well as chemo- and regioselectivities have been obtained with varying P/Rh loadings. Dissolved H2 can be monitored in solution to ensure true operando conditions without gas limitation. 31P{1H} and selective excitation 1H pulse sequences have been periodically interleaved with 1H FlowNMR measurements to detect Rh–phosphine intermediates during the catalysis. Stopped-flow experiments in combination with diffusion measurements and 2D heteronuclear correlation experiments showed the known tris-phosphine complex [RhH(CO)(PPh3)3] to generate rapidly exchanging isomers of the bis-phosphine complex [Rh(CO)2(PPh3)2] under CO pressure that directly enter the catalytic cycle. A new mono-phosphine acyl complex has been identified as an in-cycle reaction intermediate.